The present invention is directed generally to nondestructive analysis of materials and more particularly to an analysis method and apparatus which utilizes electromagnetic radiation to measure one or more constituents of a sample of material, for example, the fat content of a sample of meat.
While the non-destructive test method and apparatus of the invention is useful for determining constituent content of various constituents of many materials, the disclosure will be facilitated by addressing the problem of determining the fat to lean ratio in a meat product.
Many meats and meat products are generally evaluated nutritionally on the basis of the ratio of lean meat to fat in the product. Additionally, the prices at which such products are sold are often based at least in part on this ratio. Hence, an accurate determination of the percentage fat or alternatively, the percentage lean in a given meat product assume some economic importance for both the processor and the retailer.
Heretofore, the only reliable method of accurately determining fat or lean content of a sample has been by destructive testing. That is, a sample of the product must be fully cooked and the rendered fat product assayed to determine its percentage as a constituent of the product. Quite apparently, this destructive method of testing is not suitable for relatively frequent tests of relatively large numbers of samples of meat products. The prior art has developed an alternative, electromagnetic test method and apparatus for non-destructively performing such a measurement. One such apparatus is shown for example in U.S. Pat. No. 3,735,247 to Harker. While the method and apparatus disclosed therein has proven useful in many applications, there remains room for yet further improvement.
For example, some problems have arisen with respect to proper sample preparation to ensure both accuracy and repeatability of the measurements obtained by prior art devices. In this regard, it is important to accurately weigh each sample and accurately position the weighed sample in the prior art measurement apparatus.
Additionally, the fundamental measurement technique involved in the prior art apparatus involves inserting a sample into an electromagnetic coil which is shielded against extraneous influence. However, the shield must be open at least at one end thereof to allow entrance and egress of a sample. Such shielding has proven difficult to achieve. Moreover, for accurate operation, the measurement coil must extend well beyond the ends of the sample and the shield must extend well beyond the ends of the coil. While the latter requirement is easy to achieve, it is sometimes difficult to assure that an operator will properly position a sample within the coil.
The foregoing prior art electromagnetic test apparatus and method is based upon the differing electrical properties of lean and fat animal tissue. In this regard, primarily of interest is the relatively large ratio of conductivity between fat and lean tissues which may vary between 5 and 100. However, a ratio of between 1.1 and 10 has also been found between typical lean and fat dielectric constants. Heretofore, it has been proven difficult to separate the electrical effects upon the measurement coil of the conductivity of a sample on the one hand and the dielectric constant on the other hand. Since the relative magnitudes of these two electrical properties may vary considerably in a given sample, it is important to adequately isolate but one of these properties for measurement purposes. Since the conductivity ratio of fat to lean is typically considerably higher than the dielectric constant ratio, it is preferably to isolate the former for measurement purposes. In the prior art apparatus it has proven difficult to eliminate the effects of dielectric constant and assure measurement of substantially only the conductivity of a sample.
The prior art method and apparatus is also based primarily upon power losses and phase shifts in the sample which are reflected back to the field source in the measurement coil. It will be recognized that the power loss is related to conductivity while the phase shift is related to dielectric constant. In order to accurately measure conductivity, then, relatively small incremental power losses must be accurately measured. To ensure accuracy of the power loss measurement, the prior art apparatus required a considerable power input to the measurement coil and hence relatively bulky and expensive high-power electrical and electronic components.
When utilizing a coil as the primary measurement apparatus, additional problems are encountered with the known variations in magnetic and electric field components produced by a coil over the radius or along the axis of a sample of material placed in the coil. For example, it is known that the transverse magnetic (TM) component of the electromagnetic field is advantageous in that the electric field remains constant with changing radius. However, in the prior art apparatus it has been found that at the frequencies required for measurement (100 kHz to 10 MHz), and with the dimensions of a coil necessary to accommodate an adequate sample, it is difficult or impossible to generate a uniform TM mode. Hence, the above-mentioned prior art apparats utilizes primarily the transverse electric (TE) mode for obtaining measurements. Unfortunately, the TE mode field lines run parallel to the axis of the coil and hence the field varies considerably over the radius of a sample within the coil. Since the fat or lean content can also be fairly assumed to vary within a given sample either axially or radially, the foregoing factors present some difficulty in obtaining accurate and repeatable measurements.